On the western or seaward slopes of the Peruvian Andes the trade winds descend, and the process of rain-making is reversed to one of rain-taking. The descending air currents are compressed as they reach lower levels where there are progressively higher atmospheric pressures. The energy expended in the process is expressed in the air as heat, whence the descending air gains steadily in temperature and capacity for water vapor, and therefore is a drying wind. Thus the leeward, western slopes of the mountains receive little rain and the lowlands on that side are desert.
THE CLIMATE OF THE COAST
A series of narrow but pronounced climatic zones coincide with the topographic subdivisions of the western slope of the country between the crest of the Maritime Cordillera and the Pacific Ocean. This belted arrangement is diagrammatically shown in [75] . From the zone of lofty mountains with a well-marked summer rainy season descent is made by lower slopes with successively less and less precipitation to the desert strip, where rain is only known at irregular intervals of many years’ duration. Beyond lies the seaward slope of the Coast Range, more or less constantly enveloped in fog and receiving actual rain every few years, and below it is the very narrow band of dry coastal terraces.
Fig. 75—Topographic and climatic provinces in the coastal region of Peru. The broadest division, into the zones of regular annual rains and of irregular rains, occurs approximately at 8,000 feet but is locally variable. To the traveler it is always clearly defined by the change in architecture, particularly of the house roofs. Those of the coast are flat; those of the sierra are pitched to facilitate run off.
The basic cause of the general aridity of the region has already been noted; the peculiar circumstances giving origin to the variety in detail can be briefly stated. They depend upon the meteorologic and hydrographic features of the adjacent portion of the South Pacific Ocean and upon the local topography.
The lofty Andes interrupt the broad sweep of the southeast trades passing over the continent from the Atlantic; and the wind circulation of the Peruvian Coast is governed to a great degree by the high pressure area of the South Pacific. The prevailing winds blow from the south and the southeast, roughly paralleling the coast or, as onshore winds, making a small angle with it. When the Pacific high pressure area is best developed (during the southern winter), the southerly direction of the winds is emphasized, a condition clearly shown on the Pilot Charts of the South Pacific Ocean, issued by the U.S. Hydrographic Office.